It is generally known to utilize thermoplastic multilayer structures, such as films, sheets or the like, to package products. For example, typical products packaged with thermoplastic multilayer structures include perishable products, such as food. Specifically, meats and cheeses are typically packaged in thermoplastic structures. Moreover, shrink films are known for packaging food products, such as meat and cheese.
When meat and cheese are packaged within thermoplastic multilayer structures the moisture barrier and oxygen properties are important. The oxygen barrier is obtained by using barrier resins such as EVOH and polyamides. Multilayer films, typically made by coextrusion, frequently include a polyolefin layer such as LDPE, LLDPE, HDPE, EVA, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, ethylene-acrylic acid ester copolymers, ethylene-methacrylic acid ester copolymers, ionomers, and the like. Barrier resins used are typically ethylene-vinyl alcohol (EVOH) or polyamide resins such as nylon.
Tie-layer adhesives are used to bond polyolefins to dissimilar substrates in multilayer, co-extruded structures, particularly for blown and cast film, extrusion coating, blow molding, sheet extrusion, wire & cable, pipe, and other industrial applications. The tie-layer adhesive typically comprises a polyolefin base resin, which is the predominant component, and a grafted polyolefin. The grafted polyolefin is produced by reacting a polyolefin with an unsaturated monomer at elevated temperatures with or without a free-radical initiator. Commercially available tie-layer adhesives include Plexar® resins, product of Equistar Chemicals, LP, which are anhydride-modified polyolefins.
In addition to the base resin and grafted polyolefin, tie-layer adhesives commonly include other polymer resins or additives to provide better adhesion, clarity, or other benefits. Usually, there are tradeoffs. For instance, modifications that improve adhesion often reduce clarity, and vice versa.
Elastomers—random or block copolymers—are commonly included in tie-layer adhesive compositions to improve compatibility, increase adhesion, or impart other benefits. Styrene-based block copolymers, for instance, have been used to improve adhesion in multilayer constructions that require bonding of polyethylene to a styrenic polymer (U.S. Pat. Appl. Publ. Nos. 2007/0071988 and 2007/0167569).
Other commonly used elastomers include olefin elastomers, such as ethylene-propylene rubber (EPR) or ethylene-propylene-diene monomer rubber (EPDM). Many references teach to blend an olefin elastomer with other tie-layer adhesive components (see, e.g., U.S. Pat. Appl. Publ. Nos. 2004/0097637, 2008/0032148, 2009/0035594, and 2010/0174036), but none of these references suggests reacting the olefin elastomer with the grafted polyolefin during the manufacture of the grafted polyolefin.
Clarity of multilayer films is often an issue, particularly for the food packaging industry. Wraps for meat and cheese, snack foods, baking mixes, and many others uses require barrier layers (e.g., EVOH, polyamides, polyolefins) to prevent transmission of oxygen and/or moisture, and making such structures with high clarity poses a formidable challenge. Additives, such as the bicyclic modifiers of U.S. Pat. Appl. Publ. No. 2007/0054142, are sometimes employed to improve clarity without sacrificing adhesion.
U.S. Pat. Appl. Publ. No. 2010/0174036 teaches that tie-layer adhesives useful for multilayer films having a favorable balance of clarity and adhesion can be made by blending, in sequence, a grafted (“maleated”) polyolefin, a polyolefin elastomer, and LLDPE. Again, there is no suggestion to react the polyolefin elastomer with the grafted polyolefin.
U.S. Pat. Appl. Publ. No. 2004/0175465 teaches that multilayer structures can be made by coextrusion of several layers including tie layers. The tie layers used are anhydride modified LLDPE. Further, there is no suggestion of the tie layers contain reaction products of olefin elastomer with the grafted polyolefin.
U.S. application Ser. No. 12/924,540 discloses a graft composition useful for tie-layer adhesives and multilayer structures having improved adhesion and clarity, where the graft composition contains an LLDPE and the reaction product of a live, grafted polyolefin and an olefin elastomer.
Improved tie-layer adhesive compositions suitable for use in making multilayer structures with good adhesion and high clarity are needed. A valuable approach would avoid expensive additives and performance tradeoffs. Ideally, improved tie-layer adhesives could be made using economical starting materials, commonly used equipment, and familiar techniques. Further, coextruded multilayer structures that are oriented to provide packages that are heat shrinkable are also needed.